The hypothalamic regulation of pituitary gonadotropin secretion and sexual behavior in the male rat differs from that in the female, and these differences are established postnatally under the influence of testicular androgen. Studies will be undertaken to establish the site(s) of this permanent action of androgen on the developing brain, and two hypotheses (that androgen action alters neuronal connectivity and/or the physiology of aminergic systems) for the mechanism of androgenization will be directly challenged. In addition, two model systems in which postpubertal hormone exposure appears to have a permanent effect selectively on gonadotropin regulation (the delayed anovulation syndrome) and lordosis behavior (the septal lesioned estrogen sensitized male) will be studied in parallel. Techniques to be utilized include the radioimmunoassay of LH, FSH, prolactin, estradiol, progesterone and testosterone, the radiometric determination of biogenic amine turnover in specific brain regions, stereotaxic surgery, electrochemical and electrical stimulation of the brain of the newborn and adult animal, the recording of electrical activity from individual neurons in the newborn and adult, and an analysis of sexual behavior. This broad approach is expected to provide related and converging data to elucidate this process of the sexual differentiation of the brain.